Syrian Democratic Forces spokesman Talal Ali Sello said at a press conference that "between October 30 and November 13, an area of 1,362 square kilometres (545 square miles) was cleansed of IS".
That area included 196 towns and villages, among them Al-Hol which lies along a strategic IS supply route and where Sello made his comments.
He said these "victories" had been achieved with air support from a US-led coalition striking IS targets in Syria over the past year.
The offensive south of the city of Hasakeh, where control is divided between Syria's regime and Kurdish militia, saw the alliance take several villages before seizing Al-Hol on Thursday.
Signup to our newsletter and follow us on Facebook and Twitter!
According to Sello, 493 IS militants, 33 SDF fighters and four civilians were killed in the two weeks of fighting.
Another 53 SDF fighters were wounded.
The "balance sheet" presented in the SDF statement also said the alliance had seized hundreds of weapons and munitions and destroyed dozens of vehicles used by IS.
The SDF was formed in mid-October as an alliance between the powerful Kurdish People's Protection Units (YPG) and rebel groups including Arab and Assyrian militias.
Since it began in 2011, Syria's war has fractured into a complex array of fronts between Kurds, rebels, regime and jihadists, and has killed more than 250,000 people.